1. Field of the Invention
The present invention relates to a method for analyzing a chemical compositions of steel and a method therefor.
2. Description of Related Arts
The Composition of a steel significantly affects properties of the steel, and the composition analysis is indispensable for quality control. The oxygen blowing period in the steel making process is so short as to be about 15 minutes. At the final stage of the oxygen blowing, the composition of the molten steel is analyzed to feed back the analyzed data for controlling the composition and the temperature of the molten steel to be within a predetermined range. Therefore, a rapidity of magnitude of seconds is required for the analysis. Further, the characteristics of the steel vary with its composition and heat treatment. Based on the identified composition of the molten steel, strict conditions of heat treatment for the composition of the steel are set to produce a steel having uniform characteristics.
In the conventional method of the analysis, the main stream of analysis for a steel was an emission spectral analysis using a discharge excitation of spark or arc, which is specified in JIS-G-1253. According to the method, the spectra is obtained only after passing the discharged and emitted light through a slit, so the discharge point has to be fixed. As a result, a specimen for analysis is required to be set at a fixed discharge point. In addition, this method requires the discharge point on the specimen to be finished to a smooth and flat surface for procuring a sufficient analytical accuracy. To satisfy these conditions, the conventional method needed to cut off a small ingot of necessary size from the target ingot and to finish the surface of the specimen to a smooth and flat face. For this reason, the pre-treatment for preparing the specimen consumed a lot of work and time. With a change of steel-making technology, however, the importance of a rapid analysis by skipping such a time-taking specimen preparation has been remarkably emphasized.
One of the existing methods of the rapid analysis is an spectral emission analysis in which the emission section is separated from a spectral section and the beam generated by discharge is sent to the spectral section through an optical fiber. This method contributes merely to making the discharge point relatively free. The method, however, still requires the smooth and flat surface of a specimen and the control of discharge characteristics because these characteristics are affected by the temperature of the specimen. In addition, this method has a more serious problem of transmittance of the optical fiber to be used, and the transmittance of wave length is 200 nm or less. The wave length is so extremely low that the analytical spectra of C, P, and S which are important components of steel can not transmit through the optical fiber. This disables the analysis of these elements.
The following method solved the problems of the temperature of the specimen and of the beam transmittance. The method irradiated a high density energy to vaporize a part of a mother specimen and to collect the vaporized fine particles as a sample, and carried the sample of above the collected fine particles to an emission spectral analyzer with an inert gas. An equipment using plasma arc as a high density energy was disclosed in Japanese Patent Examined Publication No. 14773/87.
According to the mentioned patent publication, a large specimen is covered by a cylinder for generating fine particles provided with a plasma emission tube, and the specimen is heated by plasma arc to vaporize a part of it. A carrier gas is introduced to the cylinder, and an edge of the cylinder contacting the large specimen is maintained on a horizontal plane while the inside space of the cylinder is sealed to be air-tight. A vaporized part of the specimen becomes fine particles, which are then transferred by the carrier gas to the plasma emission spectral analyzer through the fine tube to be analyzed. The above described patent publication describes that high vaporization rate is the reason for selecting the plasma arc as an irradiating energy.
When the above described disclosed method is applied to an actual specimen, however, several problems remain. The one is that a difference of composition appears between the collected fine particle sample and the mother specimen, which hinders the acquisition of sufficient analytical accuracy and correct values.
another problem is that, when a specimen is a hot steel ingot, attention is not paid to handling the specimen taken from the hot steel ingot and the hot steel ingot cannot be analyzed.
That is to say, when the specimen is one having a smooth and clean surface, the analyzer is applicable, but if the specimen is taken from a hot steel ingot, the hot steel ingot has an oxide layer on a surface layer of the hot ingot and has an oxidation-affected zone such as a decarburization layer exists beneath the oxide layer. Since the composition of the oxidation-affected zone differs from the composition of the mother material, that zone should be removed. Nevertheless, the method does not consider the removal of that type of zone. If a plasma irradiation is applied to remove the zone, the peripheral area of the irradiated part melts. The melting of peripheral area easily induces an intermixing with the components of the mother material. Consequently, repeated irradiation never attains the same composition of the melted area with that of the mother material.
Furthermore, on a hot steel ingot before being subjected to rolling, the surface is not smooth and flat due to an existing irregularity called the "oscillation mark" which appears during continuous casting and is also due to the irregularity generated at cutting step. As a result, when a cylinder for generating fine particles having a flat cut end face is applied, the carrier gas leaks from a gap between the cylinder and the specimen, which results in a fluctuation of gas volume supplied to the plasma emission spectral analyzer. The fluctuation affects the excitation condition in the plasma flame, and degrades the accuracy of analysis.
On the other hand, the following alternative method is known. The method irradiates laser onto a lump sample in a sample chamber having an inert gas atmosphere and gasifies a part of the sample to generate fine particles. The fine paticles are then introduced to an IPC(Inductively coupled plasma) analyzer to emit beam or to be ionized. The method is hereinafter referred to as laser/IPC analysis. The accuracy of the carbon analysis of this method is in question, since the method has not been reduced to practice though the shortening of the operation period has been tried. For example, JP-A-No. 167446/91 disclosed a sample exchange unit which simplifies the mounting and dismounting of the sample to and from a sample chamber and the positioning by structuring a lower half part of the sample chamber to be able to slide. Since the method disclosed by the above mention publication represents an improvement from the standpoint of avoiding the cut and adjustment handling of the sample, the method contributed to a slight degree of the time-saving but failed to achieve a significant improvement in rapidity.